The present invention relates to a method for etching nuclear traces to obtain microholes having a predetermined diameter, with pluralities of nuclear traces and microholes, respectively, being produced simultaneously, for example, in a substrate such as a band-shaped foil or thin sheet.
Traditional machining techniques are incapable of producing very precise microholes, or extremely small apertures, in a substrate. Such microholes are useful, for example, in medicine, where a microhole about 5 microns in diameter can be used to determine the rigidity of individual red blood cells (diameter about 8 microns). Microholes can be produced in a substrate by irradiating the substrate with heavy ions from a particle accelerator to produce nuclear traces in the substrate, which is subsequently etched in order to develop microholes along the nuclear traces. For example, microholes can be produced in a polycarbonate foil 10 microns thick by irradiating the foil with U.sup.238 ions having a specific energy of about 1 MeV/nucleon and thereafter exposing the foil to an etchant in the for of a normal NaOH solution with 10% ethanol at 40.degree. C. The diameters of microholes depend upon a number of parameters, such as identity and thickness of the substrate, the etching time, etc.
U.S. Pat. No. 4,369,370 to Reimar Spohr, which is incorporated herein by reference, discloses a process for fabricating a single microhole produced from a nuclear trace by way of etching. However, during mass production of a band-shaped thin sheet having individualized holes with a diameter of less than a micron, it is rather time consuming to localize the individualized holes by way of a light or electron microscope so as to be able to monitor the diameter, during the etching process, and to terminate the process when the desired diameter has been reached.